Home Power Magazine - Design & Installationhttp://www.homepower.com/wind-power/design-installation
enManaging Battery Charging Using Diversion Loadshttp://www.homepower.com/articles/wind-power/design-installation/managing-battery-charging-using-diversion-loads
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Advanced</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/jeff-tobe">Jeff Tobe</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">PVcharge regulation can be relatively simple: When the batteries are full, the controller disconnects the PV array. Adding wind or microhydro to the system makes charge regulation more complicated, since turbines may overspeed if unloaded.</span></p>
<p>Often, diversion control is the solution. Off-grid “hybrid” combinations of solar, wind, or microhydro have been around for decades. These renewable electricity systems can power buildings and other sites far from the electrical grid. But hybrid systems can present unique design challenges, since multiple sources of generation—for example, a PV array, wind turbine, and a backup generator—increase the complexity of how to control battery charging so your batteries don’t get overcharged, and damaged.</p>
<p>In a PV system, the charge controller is placed between the energy source and the battery. Its job is to regulate the voltage and current coming from the energy source to charge the battery and protect the battery from overcharge (and damage). Modern charge controllers have a three-stage charge cycle. During the “bulk” phase, the voltage rises to the bulk level while the batteries draw maximum current. Once this level is reached, the absorption phase begins: the voltage is maintained at the bulk phase level for a specified time, while the current tapers off as the battery reaches a full charge. Once the battery is fully charged, the voltage drops to the float level and the battery only draws a small current until the next cycle.</p>
<p>Today’s charge controllers mostly use pulse width modulation (PWM) to control current into the battery. Pulses of current in rapid succession are allowed to pass from the energy source to the battery (or from the battery to the diversion load—more on this to follow). The controller cannot limit the size of this current, but instead controls the duration so that it can achieve the correct average current in the circuit. By modulating the width of the pulses, the controller regulates the battery charge rate. The target is to maintain the correct battery voltage for the prevailing stage of the charging cycle.</p>
<p>Some charge controllers also offer maximum power point tracking (MPPT), which no longer chains the input voltage to the output voltage. By doing so, it potentially allows more energy to be harvested from the generating source.</p>
<p>With a PV array, charge control regulation is fairly straightforward. When the batteries are full, the controller throttles the voltage/current accordingly to prevent overcharge, turning off the array in a sophisticated way. Chargers and inverter/chargers connected to generator power control the charge rate in the same way, limiting the load on the generator to regulate the charge rate. But charge control with wind and microhydro turbines is more difficult, since some turbines can overspeed when unloaded (i.e., disconnected from the battery). In an unloaded condition, a turbine will “freewheel,” increasing rpm and voltage. Excessive freewheeling can damage bearings and rotor components, and harm the electronics with the high voltage that’s produced. Turbine output must remain connected to a load at all times, yet not be allowed to overcharge the batteries. And this is most effectively accomplished with a diversion controller.</p>
<p><strong>Diversion-Load Control Options</strong></p>
<p>Diversion controllers are an effective method for managing wind or hydro turbine output and preventing battery overcharge. The turbine output is connected directly to the battery, in parallel with a diversion load controller (usually separate from any PV controller). This device shunts excess energy from the battery to a diversion (aka “dump”) load, usually a large resistor in air or a water-heating element, either of which is sized to enable constant turbine operation at its full output.</p>
<p>Some PV charge controllers can be reconfigured as diversion controllers for managing turbine output. Commonly available models range in size from 35 to 60 A for various nominal (12 V, 24 V, 48 V) battery voltages. These controllers typically allow field-selection of battery voltage; mode of operation (charge, load, or diversion control); and voltage setpoints for battery charging or load management. Often, a combination of dip switches, jumpers, and potentiometers are used to adjust these settings.</p>
<p>Programming the controller diversion setpoints is the same as setting the charging setpoints for a PV system. The goal is to set the bulk- and float-charge parameters to ensure full battery charging—without overcharge. However, if two separate controllers are being used, it may take a little time to adjust these parameters to keep the two units working together properly. It may be helpful to program the diversion controller to slightly higher or lower setpoints than the solar controller so as to encourage or discourage diversion of power to the dump load as desired. For example, a 48 V system with flooded batteries that has the PV bulk charge set to 58.4 V could have the diversion controller set to divert power during the bulk-charging cycle at 58.6 V so as to minimize diversion of PV power. Or vice-versa—for example, if we want to maximize the energy capture into the dump load for water heating. Float- and equalize-charge setpoints would be fine-tuned in a similar manner, but bear in mind that the two units may not agree on the timing of these stages of charge. The default strategy is to set the two devices to the same setpoints that best suit the battery charge regime.</p>
<p><strong>Using the Aux Output for Turbine Control</strong></p>
<p>Commonly used in PV systems, maximum power point tracking (MPPT) charge controllers may also offer options for turbine control. MPPT means that the input voltage of the controller is adjusted to maximize the PV array’s productivity. This function is independent of the actual charge control process and offers the advantages of higher-voltage transmission as well as enhanced energy production. On the charge control end of things, most PV charge controllers have at least one auxiliary output feature for diversion control of battery charge rate. Blue Sky Energy, MidNite Solar, OutBack Power, and Schneider Electric all have MPPT models with auxiliary output and programming to support diversion-load applications. This allows for the connection of turbines and/or for maximizing energy usage by diverting excess solar energy to useful heating. Blue Sky Energy offers its Duo Option Diversion Control upgrade component, which converts the auxiliary output for its Solar Boost 3024 controller into a 20 A diversion control. This unique conversion allows simultaneous MPPT operation for the PV array while also diverting up to 20 A through the converted auxiliary output. (Note this controller model is limited to 12 V and 24 V battery systems.) This can be a good solution for a smaller system, since it minimizes the space required for control equipment. Another upgrade option can increase the diversion capability to 40 A.</p>
<p><strong>Dedicated MPPT Controllers for Turbines</strong></p>
<p>MPPT voltage conversion can also be used to maximize turbine output in some cases. Some MPPT controllers can track microhydro turbine output by adjusting a few setpoints. In this application, the turbine’s DC output would be connected directly to the charge controller’s DC input, much like a PV array. MidNite Solar and Morningstar also offer wind turbine MPPT functions in some of their controllers. However, wind turbine output cannot be tracked to find maximum power. The installer must enter values or a “power curve” into the controller’s memory to suit the particular wind turbine.</p>
<p>MidNite Solar’s integrated solution couples its Classic charge controller with the “Clipper” add-on. This unique solution can be used for both wind and microhydro applications. The Clipper provides protected DC input into the charge controller, which in turn performs MPPT-like management of the turbine for maximum power output. Advantages include built-in components, such as diversion load, solid-state relay, and a run/stop breaker. The Clipper is similar to the integrated diversion controls offered with some wind turbines. Units are available in DC and AC options. AC models rectify the turbine’s wild AC output into controlled DC input for the charge controller to process. The resistor bank can be configured to match the turbine’s output, and multiple units can be paralleled for larger turbines.</p>
<p>Morningstar has a variety of controllers designed for MPPT for PV and wind systems, including the MPPT 600 V TriStar controller. This controller has a battery output rating of 60 A and also can be paralleled (up to four units) for larger wind units. However, these controllers do not have auxiliary outputs for diversion control, nor does Morningstar offer DC voltage protection add-ons. OutBack Power’s FLEXmax series (60 and 80 A models) can be used for hydro but not wind. They do have diversion features but no protection add-ons, so it is important to ensure that the turbine output cannot exceed the Voc of these controllers.</p>
<p><strong>Diversion Load Control Approaches</strong></p>
<p>The wind and/or microhydro turbine and the type of charge controller used for the PV array are two main factors in determining an optimal diversion-control strategy. For a PV array and a turbine with DC output, a common scenario is to use an MPPT charge controller for the array with an auxiliary output to control diversion of turbine energy. Used with a relay and an air-heating diversion load, this is one of the most economical, and easiest, approaches. Depending on the voltage and wattage ratings, an air-heating dump load with a wattage range between 1,000 W and 2,100 W can cost between $150 and $250; a suitable DC-rated relay may cost about $50. Mounting a prefabricated air-heating diversion load can often be much easier than retrofitting a DC circuit to a water heater tank.</p>
<p>For larger systems, multiple dump loads may be necessary to handle full output diversion from the turbine. The number of relays and the number of dump loads will depend on their power ratings. Loads can be paralleled to achieve a sufficient capacity. In smaller hybrid systems, or ones that receive only seasonal use, a separate PWM controller for managing the turbine and preventing battery overcharge is common, since these systems may not be able to fully realize the advantages of MPPT charge controllers. This system type could be designed with two separate PWM controllers—one unit manages the PV array, and another manages output from the turbine. Costs depend on the size and model. For a 24 V system with turbine output of 25 A or less, a basic PWM controller rated for 40 A with a 25 A to 40 A diversion load would cost about $300. Adding another PWM controller for the PV array brings the cost to $450—still less than most MPPT controllers, which typically start at $550.</p>
<p><strong>Diversion Load Sizing</strong></p>
<p>Properly sizing the diversion load is fairly straightforward, but important—if too small, it will not divert all of the turbine power, subjecting the battery to overcharge. If the diversion load is too big, it could overload the controller and cause it to disconnect, leading to unregulated battery overcharging. Add up all of your uncontrolled charging sources (i.e., wind turbine and/or microhydro turbine), then include a safety factor—the 2011 National Electrical Code (NEC) suggests adding a safety factor of 150%. (This factor accounts for potential spikes in output and provides longevity of the load component since, under normal turbine operation, the load would only be operating at two-thirds capacity.) Then, choose a diversion controller with a rating equal to or higher than this (see an example in the “Diversion Load Calcs” sidebar). Choose a diversion load equal or higher to this, but not any higher than the controller’s capacity.</p>
<p><strong>Using Your Diverted Energy</strong></p>
<p>Stand-alone renewable energy systems suffer a bit of a disadvantage in relation to grid-tied ones, which is that much of the energy they can produce will be wasted when the battery is full. With grid-tied systems, the excess is always exported; with stand-alone systems, you have to use it, or lose it. True conservationists will tailor their usage habits relative to the battery voltage, effectively acting as human diversion controllers, with a mission to maximize the effective use of precious energy: washing clothes, sawing firewood, running the vacuum cleaner, when the wind is blowing or the sun is blazing.</p>
<p>But not everyone wants to have their lives dictated by battery voltage. How can we automate usage and prevent wasting energy? The obvious answer is to use the surplus as a useful source of heat, which is the secondary function of diversion loads, which create heat energy from surplus electricity. Often, these devices are simple wire-wound resistors, installed in the power shed, that “dispose” of the energy surplus safely and economically. We can harvest some (or all) of this heat and avoid burning fossil fuel to heat water or to keep warm.</p>
<p>Two main types of diversion loads are air-cooled resistors and water heating elements. The advantage of air-cooled resistors is that they are always available (nobody will turn them off). But PWM-driven wire-wound resistors tend to be noisy, and their whining is usually unwelcome in a living space, so this heat is usually just “dumped” into a power shed.</p>
<p>Water heating elements in a water heater tank are another type of diversion load. They can provide water heating and perhaps space heating. If your renewable energy system is adequately sized to maintain the battery at a healthy state of charge, quite a bit of surplus heat energy will be available to divert. The disadvantage is that you cannot readily put a thermostat on these heaters because they must always be available to control battery charge, and they will be working on DC, which may damage the thermostat.</p>
<p>To use the output of the diversion charge controller directly, you will need to obtain special elements designed to work at lower voltage, or put up with much lower power output from heaters designed to work at grid voltage—for example, 120 VAC heaters will only yield one-quarter of the heating capacity if operated at 60 VDC.</p>
<p>The solutions to this problem are many and various. One option is to obtain specialized low-voltage heating elements, or use inverter power to operate grid-voltage elements via a relay. In the low-voltage case, you can use a changeover relay to shift the power to an air-heating element when the water heater thermostat opens, or you can design a water system that copes with the excess heat without the need for a thermostat. In the inverter-powered case, you can prioritize diversion to hot water via the inverter, but program a backup diversion at battery voltage that takes over after the water reaches a certain temperature.</p>
<p>Diversion control of battery charge is more than just a way to deal with renewable energy from turbines. It’s an opportunity to increase the efficiency of your energy system by automatically using any energy surplus. The amount of effort you put into this needs to be commensurate with the gains, but in the case of hydro turbines, for example, there can be a very large amount of surplus and it is well worth doing.</p>
<p><strong>Manufacturers</strong></p>
<p>Apollo Solar • <a href="http://apollosolar.com" target="_blank">apollosolar.com</a></p>
<p>APM Hydro • <a href="http://apmhydro.com" target="_blank">apmhydro.com</a></p>
<p>Blue Sky Energy • <a href="http://blueskyenergyinc.com" target="_blank">blueskyenergyinc.com</a></p>
<p>MidNite Solar • <a href="http://midnitesolar.com" target="_blank">midnitesolar.com</a></p>
<p>Morningstar • <a href="http://morningstarcorp.com" target="_blank">morningstarcorp.com</a></p>
<p>OutBack Power • <a href="http://outbackpower.com" target="_blank">outbackpower.com</a></p>
<p>Schneider Electric • <a href="http://solar.schneider-electric.com" target="_blank">solar.schneider-electric.com</a></p>
<p>Solar Converters • <a href="http://solarconverters.com" target="_blank">solarconverters.com</a></p>
</div></div></div>Sun, 01 Mar 2015 02:14:34 +0000Michael Welch13269 at http://www.homepower.comhttp://www.homepower.com/articles/wind-power/design-installation/managing-battery-charging-using-diversion-loads#commentsASK THE EXPERTS: Wind is Hardhttp://www.homepower.com/articles/wind-power/design-installation/ask-experts-wind-hard
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">The leading statement in the “<a href="http://www.homepower.com/articles/wind-power/equipment-products/2014-wind-turbine-buyers-guide" target="_self">2014 Wind Turbine Buyer’s Guide</a>” (HP161) is right on—“Without question, wind is a tough renewable energy resource to tap.”</span></p>
<p>I love the Home Power cover shots showing people strapped to a 100-foot tower while a crane, probably another 20 feet over their heads, tries to avoid dropping several hundred pounds of metal on them. Why anyone thinks this is glamorous is completely beyond me. Home-scale wind is dangerous, expensive, takes a lot of real estate, and is so much more involved than PV that—for all but the very few with unlimited time and resources—wind is a no-go. That should tell people capable of looking past the glamour to pass by wind energy.</p>
<p>Yes, I know the arguments about how wind complements PV, but at what cost? At a wind velocity of 11 meters per second, the small Kestrel puts out 1 kW, according to your comparison sheet. That’s about three PV modules—which have a fraction of the cost, little or no maintenance, and 30-plus years of output with a 25-year warranty—not a five-year wind machine warranty.</p>
<p>And why are manufacturers publishing an 11 m/s wind speed’s output—who has that kind of wind? It’s unrealistic and a sad commentary on an industry that can’t compete in the renewable energy business outside of large-scale commercial turbines. How is anyone realistically justifying wind?</p>
<p>Sorry, but this confirms my belief that wind has very little place in small-scale energy production.</p>
<p><strong>Robert Dee</strong> • via homepower.com</p>
<p>Small wind is not for the faint of heart. I talk most of my clients out of it, especially as the cost of PV modules continues to drop. For a wind-electric system to make sense, it requires a great wind resource; a dark and windy season (in the case of justifying an off-grid system); or a strong desire to just do it. It is a blast (if you like that sort of thing) to install and keep a system running, but it’s not cheap, easy, or reliable.</p>
<p>One of the presenters at the recent Small Wind Conference gave a presentation titled “Go Big or Go Home,” and I think there’s a lot of logic to that. The economics and the equipment quality both improve as machine size increases.</p>
<p>“Wind complements PV” is a reasonable off-grid approach. On-grid, it’s generally wiser to examine your resources and sink your money into generating energy with the most reliable and abundant resource—be that sun, wind, or falling water. With net metering, there’s little need to have your generating source producing evenly all year. PV can make most of your energy in your sunny season (your utility credits the surplus to your account), and then you can draw on the credit during times of lower production.</p>
<p>I agree that the 11 m/s value is a bit high for a rating—but that is an instantaneous wind speed, not an average. And any instantaneous rating is pretty useless for comparison with PV or with other machines, and for energy predictions. What’s really helpful is an energy rating at various average wind speeds, as shown in the article’s table. Then you can (with luck) find the average wind speed at tower-top height at your site and get a prediction of the kilowatt-hours a given machine may provide each year.</p>
<p>A “1 kW” machine and 1 kW of PV are not comparable. A 1 kW PV system rarely produces at full power, but has a fairly predictable energy (kWh) output if you know the peak sun-hours at the location. A wind turbine rated at 1 kW peak is not similarly predictable, since wind is a cubic resource. For example, cutting the wind speed in half yields about one-eighth the potential power. You’ll need to know the actual tower-top average wind speed to make a reasonable energy prediction.</p>
<p>You are wise to point out that the turbine cost is just one part of the system’s cost. Typically, it’s a small portion—in most cases, the tower and balance-of-system components each cost more than the turbine. Potential wind energy users need good pricing on the installed cost of all of the components before deciding to invest in a wind-electric system. Some will go for it regardless of the economics. In all cases, it’s wise to know the costs and the benefits.</p>
<p><strong>Ian Woofenden</strong> • <em>Home Power</em> senior editor</p>
</div></div></div>Tue, 30 Dec 2014 20:57:38 +0000Michael Welch13176 at http://www.homepower.comhttp://www.homepower.com/articles/wind-power/design-installation/ask-experts-wind-hard#commentsMAILBOX: Wind Physicshttp://www.homepower.com/articles/wind-power/design-installation/mailbox-wind-physics
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>I enjoyed David Laino’s article on wind physics (“<a href="http://www.homepower.com/articles/wind-power/design-installation/wind-energy-physics">Wind Energy Physics</a>” in HP161). It has excellent, usable, and applicable physics information that dispels a lot of common mistakes that the inexperienced would make.</p>
<p>I’m a 69-year-old solar enthusiast who’s been into solar since the early 1970s (mostly PV and hot water systems—my latest project is eight solar hot water collectors to heat my greenhouse). My first wind generator attempt would have been to build a funnel for more air through the machine. Multi-bladed machines came to mind, followed by different designs such as some of the illustrations you provided—all mistakes you helped me avoid! Thanks for helping me and others see the errors of our thoughts.</p>
<p>I’ve gotten some bad information in the past, and it just cost me a lot of time and money. I want to compliment you on a well-written article—clear, concise, precise.</p>
<p>Don Tollefson • Venice, California</p>
</div></div></div>Tue, 30 Dec 2014 02:21:34 +0000Michael Welch13172 at http://www.homepower.comhttp://www.homepower.com/articles/wind-power/design-installation/mailbox-wind-physics#commentsASK THE EXPERTS: Wind Speed Cubedhttp://www.homepower.com/articles/wind-power/design-installation/ask-experts-wind-speed-cubed
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Advanced</li></ul></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">When I was in fifth grade, I was told that doubling wind speed quadrupled the force of the wind. </span>However, I’ve recently learned that the wind’s power is proportional to the cube of its speed, rather than the square. The first equation in David Laino’s <a href="http://www.homepower.com/articles/wind-power/design-installation/wind-energy-physics">article</a> on wind physics in <em>HP161</em> says that kinetic energy equals 1/2 times mass times velocity squared, which raises two issues. First, the article doesn’t explain why the velocity is squared, which seems necessary for following the rest of the logic leading to the cube-of-velocity concept.</p>
<p>Second, the coefficient 1/2 presumably requires that specific units be used, such as kilograms and meters. Can you better explain these?</p>
<p><strong>Malcom Drake</strong> • via email</p>
<p>What you learned in fifth grade was correct: The force of the wind is a function of the wind speed squared. However, force and pressure are static measures, whereas power and energy are dynamic measures. It is important not to confuse them. Static does not consider motion and dynamic does. Thus, it makes sense that the addition of motion—velocity—to a static value that already has a squared dependency on velocity yields a cubic dependency on velocity.</p>
<p>A presentation of the concepts in the article required accepting the laws of physics at some level. The details of the derivation of the kinetic energy equation were outside the scope of the article, but if we accept Newton’s second law of motion—that force equals mass times acceleration (F = ma), and the definition that energy (E) is the integration of force acting over a distance (s)—then we get this calculus equation:</p>
<p>E = ∫F × ds = ∫m × a × ds = m∫ (dv/dt) × ds = m∫ (ds/dt) × dv = m∫ (v) × dv,</p>
<p>which leads to 1/2mv<sup>2</sup></p>
<p>If you are familiar with integral calculus, you’ll recognize that both the 1/2 and the squared terms are the result of integration, with the result that E = 1/2mv<sup>2</sup>. Thus the 1/2 term is not a coefficient for specific units—you are free to use any units you choose. If you use SI units (kg, m, and sec) you will get an answer for energy directly in watt-seconds.</p>
<p>The velocity-cubed function of kinetic energy is unique to continuous fluid flow. It also applies to water flow because the mass of the fluid passing through an area depends directly on how much fluid—and how fast that fluid—is flowing through the area. This is not true for the kinetic energy of a solid mass, such as a baseball. The baseball’s mass is a constant and thus its energy is only a function of the velocity squared. So take advantage of the power of the cubic by choosing wind power over baseball power!</p>
<p><strong>David Laino</strong> • Cofounder, Endurance Wind Power</p>
</div></div></div>Fri, 31 Oct 2014 19:11:46 +0000Michael Welch12845 at http://www.homepower.comhttp://www.homepower.com/articles/wind-power/design-installation/ask-experts-wind-speed-cubed#commentsNEC Calculationshttp://www.homepower.com/articles/solar-electricity/design-installation/nec-calculations
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Advanced</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/ryan-mayfield">Ryan Mayfield</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">“Code Corner” in <em>HP159</em> and <em>HP161</em> detailed the requirements for circuit sizing and overcurrent protection under the 2014 National Electrical Code (NEC). Here are some example calculations to bring that Code language to life. </span>For those who use the 2011 NEC, don’t worry—the calculations are the same.</p>
<p>If you’re rusty on the “whys” behind the calculations, consider reviewing those previous “Code Corner” articles to understand the background information. The calculation examples use:</p>
<ul><li>72-cell, 310 W module; 45.4 Voc, 9.28 A Isc, and 15 A maximum series fuse rating</li>
<li>Four PV source circuits connected to a single string inverter</li>
<li>The location is Sacramento, California, with an ASHRAE 2% average high temperature of 38°C</li>
<li>Exposed rooftop conduit is 2 inches above the roof surface<br />
The effective temperature inside raceway [from Table 310.15(B)(3)(c)] = 38°C + 22°C = 60°C<br />
The 90°C conductor insulation correction factor for 60°C from Table 310.15(B)(2)(a) is 0.71</li>
<li>The temperature of conductors that run through the attic space is estimated at 55°C<br />
The 90°C conductor insulation correction factor for 55°C from Table 310.15(B)(2)(a) is 0.76</li>
<li>All terminals located in junction and combiner boxes are rated for 75°C</li>
<li>All exposed conductors are PV wire and rated at 90°C; all conductors within raceways are THWN-2 and rated at 90°C</li>
<li>With four source circuits, there are eight current-carrying conductors in the raceway, and the correction factor is 70% from Table 310.15(B)(3)(a).</li>
</ul><h2>Independent Source Circuits</h2>
<p>For our first example, all PV source circuits run independently from the modules to a rooftop junction box (no combining on the roof) and then continue down to the inverter. The raceway is on the rooftop and exposed to sunlight, and runs through the attic before terminating at an inverter inside the building that contains an integrated combiner and DC disconnect. This scenario—keeping the source circuits independent from the array to the inverter—is common in residential applications.</p>
<p>We can jump straight into the circuit-sizing section of the NEC. First, find the maximum circuit current:</p>
<ul><li>From Section 690.8(A): Imax = Isc × 1.25 = 9.28 A x 1.25 = 11.6 A</li>
</ul><p>Next, determine the minimum conductor size required:</p>
<ul><li>Under 690.8(B)(1): Icont. = 11.6 A Imax x 1.25 = 14.5 A</li>
</ul><p> From Table 310.15(B)(16): Under the 75°C column, 14 AWG copper is the smallest conductor that exceeds the 14.5 A requirement (see “Code Corner” in HP159 for discussion of the terminal limitations and 75°C lookup).</p>
<p>The second half of 690.8(B) is required to confirm the conductor chosen in (B)(1) has enough ampacity when exposed to the conditions of use for that circuit. The conditions of use will be the temperature the conductors are exposed to, as well as the number of conductors in the raceway. There are three different temperature conditions the circuit is exposed to: rooftop, attic, and building interior. Section 310.15(A)(2) requires that “where more than one ampacity applies for a given circuit length, the lowest value shall be used.” This means, based on the general rule, we are required to use the circuit section exposed to the highest temperature to determine the proper conductor sizing.</p>
<p>Reading 310.15(A)(2) in its entirety reveals an interesting exception when two different ampacities apply to adjacent portions of a circuit. If the condition that results in the conductor’s lowest ampacity is no greater than 10 feet and not more than 10% of the total circuit length, we are allowed to use the higher ampacity (lower temperature) calculation. This is due to the ability of the circuit to dissipate the heat for the relative short run through the higher-temperature location.</p>
<p>This exception affects the example if the rooftop raceway is run for a length less than 10 feet and that distance is also less than 10% of the total circuit. Our example total circuit length is 55 feet, and the exposed rooftop raceway is 9 feet, so we cannot apply the exception. Our lowest ampacity condition will be in the rooftop raceway section.</p>
<p>To confirm that the conductor has the appropriate ampacity under conditions of use per 690.8(B)(2), apply the correction factors (listed above) to the conductor’s actual ampacity from the 90°C column:</p>
<ul><li>25 A 90°C ampacity × 0.71 Table 310.15(B)(2)(A) correction factor × 0.7 Table 310.15(B)(3)(a) factor = 12.4 A</li>
</ul><p>Using 14 AWG is permitted because the ampacity with conditions of use applied is greater than the Imax value. The final step is to consider the overcurrent protection devices (OCPDs) and verify that the conductors will be properly protected. Since there are more than two source circuits, the exception in 690.9(A) won’t apply; OCPDs will be required. The minimum size will be calculated as:</p>
<ul><li>690.9(B): Imax × 1.25 = 14.5 A; so the OCPD size will be 15 A.</li>
</ul><p>But will the 14 AWG conductors be properly protected by a 15 A fuse? Article 240, as referenced by 690.9, provides allowances for an OCPD rating greater than the conductor’s ampacity. In this case, the conditions of 240.4 are met, so a 15 A OCPD is allowed. Even with relatively strict applications of correction factors, a 14 AWG conductor would still be considered Code-compliant for this installation.</p>
<h2>Rooftop Combiner</h2>
<p>For this example, a rooftop combiner box is installed in place of the junction box. NEC requirements such as fuse servicing and disconnecting requirements make this less common in residential applications; but it’s more common in commercial installations. We size the two current-carrying conductors leaving the rooftop combiner and running into the inverter’s DC disconnect.</p>
<ul><li>690.8(A)(2): Imax = 4 x 9.28 x 1.25 = 46.4 A</li>
<li>690.8(B)(1): Icont. = 46.4 A x 1.25 = 58 A; means 6 AWG copper</li>
<li>690.8(B)(2): 75 A x 0.71 x 1.00 = 53.25 A; which is greater than 46.4 so 6 AWG works.</li>
</ul><p>Finally, if an OCPD is used in the output circuit, confirm the required size and verify the conductor’s ability to carry the current:</p>
<ul><li>690.9: 46.4 A x 1.25 = 58 A; so a 60 A OCPD required</li>
<li>Per 240.4, a conductor with 53.25 A of ampacity is properly protected by a 60 A OCPD; again, 6 AWG is OK.</li>
</ul><p>With practice and patience, understanding the rationale behind the calculations and the calculations themselves will become easier.</p>
</div></div></div>Sat, 30 Aug 2014 21:09:34 +0000Michael Welch12800 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/nec-calculations#commentsInverter & Battery Cableshttp://www.homepower.com/articles/solar-electricity/design-installation/inverter-battery-cables
<div class="field field-name-field-subtitle field-type-text field-label-hidden"><div class="field-items"><div class="field-item even">Make the Right Connections for Best System Performance</div></div></div><div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/carol-weis">Carol Weis</a></div><div class="field-item odd"><a href="/profiles/christopher-freitas">Christopher Freitas</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">Poor-quality and improperly installed battery and inverter cables can cause problems in the function and safety of a battery-based system. Here’s how to select the right cables and install them correctly, for optimal system performance.</span></p>
<p>There is a perception that battery and inverter cables are expensive—and it is a tempting place to cut costs—but buying cheap cables can result in significantly reduced performance of the battery bank and inverter(s). It’s a lot like putting cheap tires on a high-performance car—you save some money, but you don’t get the performance and safety you might need. The common problems seen with cabling in battery-based renewable energy (RE) systems are typically due to low-quality cables and hardware, in combination with poorly made crimps and connections.</p>
<p>You can purchase preassembled cables or have them made to order, but you can also build them yourself. The details are important—battery cables and their ring terminal connectors (also called “lugs”) carry high current and are used in harsh environments where they can be exposed to sulfuric acid, hydrogen gas, high temperatures, and dissimilar metals.</p>
<h2>Cable Ampacity</h2>
<p>For battery/inverter RE systems, the largest conductors in the system are usually the ones connecting all of the batteries together and then exiting the battery box to connect to the inverter. Since nearly all battery-based inverters operate at 48 VDC or lower, the cables need be large to handle high currents without significant losses. Sizing of these cables is based on the battery voltage, the inverter’s continuous amperage rating, and the length of the cable. Commonly, these cables are either 2/0 AWG (acceptable for use with a maximum of 175 A breaker or fuse) or 4/0 AWG (acceptable for use with a maximum of 250 amp breaker or fuse), but will need to be individually calculated. For example, the installation manual for OutBack Power Systems’ VFX3524 (3,500 watts; 24 VDC) inverter recommends 4/0 AWG for a battery-to-inverter cable length of 10 feet or less. This size cable would result in a voltage drop of less than 1% at full rated output of the inverter, resulting in 34 watts of losses in the 10-foot-long positive and negative conductors. Shorter cables would reduce the losses proportionally. </p>
<h2>Cable Types</h2>
<p>High-quality battery/inverter cables are made of fine-strand copper conductors with a flexible insulation covering and are available from manufacturers such as Polar Wire Products or Cobra Wire &amp; Cable. Although finely stranded cables are not required, they make installing and servicing the system easier and reduce stress on the battery and inverter terminals. All high-quality battery cables are made with UL-listed wire and include a National Electrical Code (NEC)-required designation, such as RHW, THW, or THHW. </p>
<p>Lower-quality battery cables are often made from automotive or welding conductor cable. This type of cable is cheaper and easier to obtain—but is not acceptable by the NEC since it is not UL-listed or marked with the NEC wire type. While some types of welding cable have a UL listing, they have been approved using a different set of UL standards and tests, and are not marked with the required NEC wire-type designation.</p>
<h2>Lugs</h2>
<p>There are many different types of battery cable lugs to choose from if you’re making your own cables. The following are things to consider:</p>
<p><strong>Material</strong>. Lugs can be made from many different materials, including copper, steel, or aluminum. To establish high-quality, long-lasting connections, only copper lugs are recommended. Steel or aluminum lugs will corrode over time from environmental conditions or from galvanic corrosion that occurs when dissimilar metals come into contact.</p>
<p><strong>Bare or tin-plated</strong>. Copper lugs come in two varieties: bare or tin-plated. The tin-plated copper lugs are usually a dull gray color, and are preferred, especially for use at battery connections, since the plating reduces corrosion that can occur between the copper lug and the battery’s lead terminals, especially when there is battery acid involved.</p>
<p><strong>Open- or closed-ended</strong>. To prevent corrosion from entering the conductor strands, use closed-ended lugs at the battery. Open-ended lugs are less expensive and more commonly available, and can be used when making inverter and breaker connections, but should not be used for the battery connections.</p>
<p><strong>Listing</strong>. A lug needs to be tested and approved to UL standards, and rated for the system’s maximum voltage, current, temperature, and conditions of use. When using a fine-strand cable, you’ll also need to select lugs that are listed for the wire type (for example, fine strand wire is typically Class K). This can be difficult to determine or verify from the information available from the lug markings and manufacturer’s datasheets.</p>
<p><strong>Sizing</strong>. All crimp-type lugs are rated to fit a specific conductor size, so matching the lug to the conductor is required by the NEC to ensure a good connection. </p>
<p><strong>Bolt-hole diameter</strong>. It is also important to choose the correct bolt-hole diameter for the lug and terminal hardware combination. Drilling out the lug’s hole to accommodate a larger bolt is not acceptable as it may reduce the lug’s current rating; it also can result in higher resistance and excessive heat buildup that could potentially result in melted battery terminals or a fire.</p>
<p><strong>Ring terminal size &amp; shape</strong>. The “flag” or “ring” of the lug that attaches to a terminal comes in a variety of shapes and sizes. Lugs that provide a large surface area reduce resistance and the possibility of digging into soft lead battery terminals. Some breakers and inverters may need a smaller-size ring to fit on their terminals.</p>
<p>Avoid using set-screw-type compression lugs with finely stranded cable. Under pressure, the fine strands can twist and break off. The high number of strands makes the flexible cable’s connection “soft,” resulting in a connection that will be difficult to get tight and could potentially become loose over time. </p>
<h2>Tight Lug &amp; Cable Connections</h2>
<p><strong>Crimping</strong>. When lugs are not securely crimped on a cable, the loose connection causes higher resistance to the flow of electrons during charging and discharging. This can devastate the performance of a battery string or entire bank and could possibly result in melted battery terminals and even fire—but those aren’t the only problems. Additionally, this is a hazard to someone maintaining the battery bank. During a routine inspection—for example, when the electrolyte level is being checked—a cable could accidentally be bumped loose from a lug and touch something else, causing a short-circuit or shock. If there is hydrogen gas present, a resulting spark could be very dangerous. Safety concerns, as well as possible performance degradation, can be eliminated by well-made cable crimp connections.</p>
<p>A low-quality crimping tool used to compress the lug onto the cable can result in a loose connection, with only a portion of the cable’s strands of wire making electrical contact. These cheap crimpers often use a single “pin” to press on the lug’s barrel and press the other side into a V-shaped groove, leaving voids inside of the lug. Over time, this poorly crimped lug will become loose, often overheating and failing. The lug may even come off the cable entirely if pulled.</p>
<p>High-quality crimpers use specific jaws to accommodate different-sized cables and compress the lug’s barrel from multiple angles, either into a square or even a hexagonal shape. This produces a much tighter connection without any voids—making sure all of the cable’s outer wire strands make contact with the lug. These types of tools are more expensive but necessary for making proper connections.</p>
<p><strong>Soldering </strong>is an additional means of sealing the connection, so even if you’re using solder-type lugs they first must be crimped on properly and then soldered. Few installers have the equipment to properly solder a cable connection without damaging the insulation; therefore, it is rarely done. Soldered connections are acceptable under the NEC, but may require additional scrutiny by an inspector to verify what lugs and processes were used. </p>
<p><strong>Protecting the cable</strong>. To further protect the battery cable strands from corrosion, seal the crimped connection with adhesive-filled heat-shrink tubing. This is available in a variety of sizes and colors from battery suppliers. It usually is made from a thick-walled plastic material and gives additional support to the delicate, fine-stranded wire where it connects to the cable lug. Don’t be tempted to use electrical tape—it is not as effective.</p>
<h2>Terminal Connections</h2>
<p>Making a secure, low-resistance connection to the battery or the inverter terminal is just as important as properly securing the cable to the lug. Use the right type and size of stainless steel hardware when attaching to the lead post which is quite soft and can be easily damaged. The correct washer, lock washer, and nut placement is critical to the connection staying tight. Thoroughly clean the lead battery terminal with a wire brush before attaching the lug to achieve a good connection. Then tighten the terminal’s hardware to the battery manufacturer’s specifications and add an anticorrosion coating. Do not put any anticorrosion coating between the terminal and the cable lug.</p>
<p>Be diligent about placing the hardware in the correct order. A hazardous condition can be created if a washer is placed between a cable lug and the inverter or battery terminal. In this scenario, the high current that is drawn by the inverter would have to pass through the washer, causing the connection to overheat—which can damage the battery or inverter terminal and even cause a fire. </p>
<h2>Cable Protection</h2>
<p>In most battery–inverter systems, the battery cables are routed from the battery enclosure into a DC disconnect enclosure, through a breaker, and then to the inverter. The NEC requires that exposed conductors be protected. A common solution is to route the cables through conduit, which comes in many different sizes and types, including flexible or rigid, and metallic or nonmetallic.</p>
<p>The cables leaving the battery bank are usually not protected from overcurrent until they are connected to the DC disconnect enclosure, making them a substantial hazard if they are not well-protected. It is recommended to use nonmetallic conduit for these circuits to eliminate the potential for ground faults. The conduit should be attached to the battery and DC disconnect enclosures using a threaded male adapter; use plastic bushings on these sharp threads to protect the cable insulation. The conduit also needs to be well-supported and attached to walls or supports to prevent it from breaking and pulling on the conductors.</p>
</div></div></div>Sat, 30 Aug 2014 20:15:43 +0000Michael Welch12798 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/inverter-battery-cables#commentsASK THE EXPERTS: Average Wind Speedhttp://www.homepower.com/articles/wind-power/design-installation/ask-experts-average-wind-speed
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Advanced</li></ul></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">It’s common to read about “average wind speed” when deciding whether or not it is worth installing a wind generator at a given site. But it seems unrealistic to talk about average wind speeds, as such. </span>Obviously, if you had winds of, say, 25 mph for half a given amount of time, and 5 mph the other half, the power output would not be the same as if the wind were a constant 15 mph—there would be 125 times as much (theoretical) power generated during the time the wind was at 25 mph than during the time it was blowing at only 5 mph. So shouldn’t this be taken into account when analyzing a potential wind site?</p>
<p>Furthermore, does it make sense to you that a wind system at my home could theoretically generate as much power in one or two days of 40 to 50 mph winds than could be generated over the rest of the year’s average winds of less than 5 mph? After all, a 50 mph wind would generate (again, theoretically) 1,000 times the power that a 5 mph wind would generate over the same time span.</p>
<p>Malcom Drake • via email</p>
<p>You are absolutely correct that average wind speed alone does not provide enough information to determine energy potential. What is required is the distribution, which specifies the fraction of the time the wind speed spends in a particular range. Because it takes years of recorded data to provide an accurate distribution for a site, assumptions are often made. The typical assumption (based on much experience and measurement) is that the wind speed follows a predictable distribution—a common one that is fairly accurate for most sites is the Rayleigh distribution. For small wind turbines, a Rayleigh distribution is what the Small Wind Certification Council uses for the certified annual energy production curve based on the certified power curve. If an average wind speed is provided with no other context, the assumption is most likely a Rayleigh distribution.</p>
<p>As for building a turbine for rare, high winds—the economics are likely unworkable. The unit would have to be very stout—and hence prohibitively expensive—and it would sit idle most of the time, as the startup speed would be high due to its mass and inefficiency in light wind. When the winds did blow fast enough, it would indeed produce a lot of power, but then you’d have to find a way to store the energy. Energy is power multiplied by time: a large amount of power for a small period of time could be equal to small amounts of power for longer periods of time, and a wind turbine designed for the latter scenario would be much cheaper.</p>
<p>David Laino • Cofounder, Endurance Wind Power</p>
</div></div></div>Thu, 28 Aug 2014 21:50:02 +0000Michael Welch12771 at http://www.homepower.comhttp://www.homepower.com/articles/wind-power/design-installation/ask-experts-average-wind-speed#commentsMAILBOX: Utility Policyhttp://www.homepower.com/articles/solar-electricity/design-installation/mailbox-utility-policy
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Beginner</li></ul></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">I would like to see an article comparing the major utilities’ policies toward renewable energy (RE). I am motivated out of disgust about the attitude of my own provider (Mississippi Power), which requires paying fees for a net usage meter of $0.53 per day!</span> I find this outrageous—some simple arithmetic will show that about 1 kW of additional PV capacity would be required to offset that cost. For example, to install a grid-connected PV system on my home, I would have to size it first to my anticipated usage, and then add another 1 kW of capacity just to offset the cost of meter rental!</p>
<p>My state has absolutely no incentives for RE installations, and although Mississippi Power is a member of the TVA Network, they choose not to participate in any of TVA’s incentive programs. A particular kicker is that I live in southwest Mississippi, only about 5 miles from the Louisiana border, and Louisiana has some of the nation’s best incentives. I guess I’m curious whether other utilities around the nation have similar policies, and if they do, what might be done about it. I realize that there are hundreds of power suppliers and an article encompassing all of them would be enormous, but a study of the major providers and their policies would certainly be well-received, particularly by those of us who are adversely affected by their utilities’ greedy policies.</p>
<p>Frederick Smallwood • via email</p>
</div></div></div>Thu, 28 Aug 2014 20:35:57 +0000Michael Welch12767 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/mailbox-utility-policy#commentsAssessing Wind: Determining Your Wind Resourcehttp://www.homepower.com/articles/wind-power/design-installation/assessing-wind-determining-your-wind-resource
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/mick-sagrillo">Mick Sagrillo</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">You want a wind system, and have researched turbine, tower, and installation companies. But one critical question remains: Will it generate the energy (kWh) that you need? To answer, you need to know the average wind speed at the tower height required for your site.</span></p>
<p>Arriving at a wind speed number is not easy, and getting it wrong can have serious ramifications, since a wind turbine’s output is related to the cube of the wind speed. For example, an 8 mph wind will yield only half the energy that a 10 mph wind will. You need a good idea of how much “fuel” you might have.</p>
<p>A wind site assessor will be able to best estimate the average wind speed at your site. Expect to pay $200 to $600 for a boots-on-the-ground visit followed by an in-depth report. While this might seem like a lot of money, consider the risk of putting up a $30,000 to $80,000 wind system without full knowledge of the resource. Even those who decided against a wind system concur that a site assessment is money well spent.</p>
<p>Online site assessments for comparable prices are available, but they are less reliable than a personal on-site visit. Some manufacturers offer online wind estimates based on your address, but with a financial stake in your decision to purchase, most of these “assessments” are too optimistic.</p>
<p>Before you hire an assessor, there is a do-it-yourself method to get a ballpark wind speed. This will help you determine if you should even consider a wind system, or if hiring an assessor is justifiable. First, consult the up-to-date wind resource maps at Wind Powering America (<a href="http://bit.ly/WindMaps" target="_blank">bit.ly/WindMaps</a>). Click on your state and view a model of the average wind resource for your “area.”</p>
<p>These maps not site-specific, but rather a two- by two-kilometer resolution interpolation. The map cannot give the wind speed at your required tower height (at least 30 feet above anything within 500 feet and the mature tree heights in your area). To fine-tune this number, you’d also need to know your tower site’s surface-friction coefficient, turbulence intensity, prevailing wind direction, and displacement height. These are the four critical factors you need to understand to optimize your site’s wind speed.</p>
<p>To better understand all of this and why it’s relevant, visit smallwindtraining.org. The “Site Assessor” tab is a resource for site assessor training workshops. There’s a lot to read, after which you’ll have a good idea of where and how to site your system. Finally, ask nearby wind system owners about their experiences—and what they might do differently.</p>
<p>—<strong>Mick Sagrillo</strong></p>
</div></div></div>Sun, 27 Apr 2014 00:09:40 +0000Doug Puffer12415 at http://www.homepower.comhttp://www.homepower.com/articles/wind-power/design-installation/assessing-wind-determining-your-wind-resource#commentsWind Energy Physicshttp://www.homepower.com/articles/wind-power/design-installation/wind-energy-physics
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/david-laino">David Laino</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in"><strong>Wind turbine design</strong> encompasses multiple disciplines, but perhaps the most important—and often the least understood, even by would-be turbine designers—is fluid dynamics. Understanding the physics laws that govern wind power, presented here, can help consumers make wise wind turbine technology choices.</span></p>
<h2>The Energy Equation</h2>
<p>The purpose of a wind turbine is to convert kinetic energy (energy of a moving mass) of the wind into electrical energy. Energy <em>conversion</em> is common to all machines because they must obey the law of energy conservation—energy cannot be created or destroyed, but only changed from one form to another. For example, your car converts the chemical energy stored in fuel (if it’s an electric car, batteries) to kinetic energy, moving it down the road. A wind turbine also obeys this law when it extracts the kinetic energy in the wind and converts it to electrical energy.</p>
<p>The amount of kinetic energy in any moving mass is calculated with this equation:</p>
<p><strong>Kinetic energy = 1/2 × mass × velocity<sup>2</sup></strong></p>
<p>For wind energy, the velocity in the equation is wind speed. The mass is for a particular volume of air. Consider the example of wind blowing through an open window. The illustration describes how a volume of the air passing through the window relates to window area, wind speed, and time. This makes sense if you consider that the larger the opening, the harder the wind is blowing, and the longer the window is open, the more air volume will flow through it.</p>
<p>The mass of this volume of air is arrived at by multiplying the volume by the air density. Putting this all together, we write our equation for wind energy as:</p>
<p><strong>Kinetic energy = 1/2 × air density × area × wind speed<sup>3</sup> × time</strong></p>
<h2>Power in the Wind</h2>
<p>Although a wind energy system’s final objective is to generate energy, it is more convenient to describe its size in terms of power. The relationship between power and energy is a simple one—energy is power multiplied by time. This is why power is defined in watts and energy in watt-hours. The terms <em>power</em> and <em>energy</em> are often confused and even used interchangeably in casual discussions, but in a technical analysis, it is important to make the distinction.</p>
<p>If we are interested in the power in the wind, we divide the energy by time. This gives us the governing equation for power available in the wind passing through an area as:</p>
<p><strong>P<sub>wind</sub> = 1/2 × air density × area × wind speed<sup>3</sup></strong></p>
<p>We see that the amount of wind power is dependent on three variables. The first is <em>air density</em>, a quantity defined by Mother Nature, over which we have no real control. Another is <em>swept area</em>, that is, the projected area perpendicular to the wind that the turbine intercepts. One thing is clear from the equation: everything else being equal, a larger swept area can generate more power.</p><p>The third (and most important for siting wind turbines) variable is <em>wind speed</em>. You can see that it’s cubed in the equation, meaning small changes in wind speed yield larger changes in available power. A 26% increase in wind speed (from 10 to 12.6 mph) doubles the available energy, while a 20% drop (from 10 to 8 mph) cuts it in half. This is why it is critical to put a wind turbine on a tall tower where it can intercept strong wind.</p>
<h2>Wind Turbine Job Description</h2>
<p>As a wind turbine extracts kinetic energy from the wind, it does not consume air mass (only nuclear reactions consume mass), so it must be “consuming” the wind speed. In other words, the wind approaches the turbine at one speed and leaves at a lesser speed. This is how any wind turbine extracts energy from the wind—by slowing it down. The difference between the wind speed before and after it passes through the turbine defines the energy the turbine has extracted from the wind. This is the fundamental function of the wind turbine, and some turbines do it better than others.</p>
<h2>Wind Turbine Efficiency &amp; Limits</h2>
<p>It is not feasible to extract all of the power available in the wind—and no wind turbine can harness more energy than is available in the wind. Avoid any wind turbine that claims it can—no wind turbine can slow wind down to a speed of less than zero.</p>
<p>If a wind turbine were to extract all the available power in the wind—that is, slow the wind to a stop and capture all its power—we would say that turbine is 100% efficient. However, any wind turbine that did this would stop the wind, and then there would be no air movement from which to extract more power! An effective wind turbine must find a balance, slowing the wind enough to maximize power capture, yet still allow enough wind to pass through so it can keep capturing more. </p>
<p>Efficiency is defined as the ratio between the output power and the input power. For wind turbines, aerodynamic efficiency is referred to as the power coefficient, C<sub>p</sub>, so the governing equation for wind turbine power output is:</p>
<p><strong>P<sub>output </sub>= 1/2 × air density × area × wind speed<sup>3</sup> × C<sub>p</sub></strong></p>
<p>We know C<sub>p</sub> cannot be 100%, but what is the upper limit? In 1919, German scientist Albert Betz took the above governing equations for wind power, and used them to determine how much a theoretically “perfect” wind turbine could extract from the wind. His answer, which is referred to as Betz’ Law or the Betz limit, states that when the wind is slowed by two-thirds (wind speed out = 1/3 wind speed in), the wind turbine reaches its theoretical maximum possible efficiency, C<sub>P</sub>-max., of 59.3%.</p>
<p>It is critical to note that Betz’ Law is derived from the governing equations, and not from any assumptions about wind turbine type. Therefore, the Betz limit is not restricted to any particular type or style of wind turbine, as some people mistakenly believe. The Betz limit is a physical limit that applies to <em>all</em> wind turbines.</p>
<h2>Wind Turbine Types</h2>
<p>In the real world, no wind turbine can ever even reach—let alone exceed—the Betz limit. But which turbines do the best, and how well do they do?</p><p>Harnessing wind power is an endeavor humans have been undertaking for thousands of years, so not surprisingly just about every type of machine design has been attempted. Some of the more common types are the Savonius, Darrieus, Dutch windmill, American multiblade (water-pumper), and modern propeller styles. The efficiencies of these machines has been determined by theory and confirmed by experience (see “Turbine Types &amp; Typical Power Coefficients” table).</p>
<p>The propeller-style wind turbine achieves the best efficiency, so it is not surprising that this is the most commercially successful wind turbine type. Engineers use the superior efficiency of the propeller wind turbine, and combine it with a large rotor area—the other governing parameter that they have direct control over—to optimize performance of their designs.</p>
<h2>“Breakthroughs” &amp; “New” Concepts</h2>
<p>There is a persistent myth that wind power is a new technology ripe for innovation. This thinking leads to wild claims of doubling (or more) the efficiency of “old” propeller designs. This, frankly, is impossible. At 45% aerodynamic efficiency, modern propeller designs are already achieving more than 75% of the theoretical Betz limit, leaving little room for “breakthroughs” in the aerodynamic efficiencies of wind turbines. Using our understanding of wind turbine fluid dynamics, we can expose some common wind turbine hype.</p>
<p>It can be counterintuitive that a propeller-style wind turbine rotor achieves the best efficiency. After all, those three (typically) thin blades let so much of the wind pass between them. Wouldn’t more blades help? That seems logical on the surface, and leads some designers to include an array of blades. This idea, however, ignores the laws of fluid dynamics. Remember, the wind turbine must strike a balance between slowing the wind and allowing it to pass. And it turns out that fewer, thinner blades spinning fast does this best. The American multiblade is effective at what it does—mechanically pumping water—but efficiency is not its goal.</p>
<p>“Augmenters” attempt to leverage the cubic function of wind speed in the energy equation by speeding up the wind into the rotor through ducts or shrouds. This method is akin to the nozzle on a hose that effectively turns a fat, trickle of water into a forceful, faster stream. The difference with wind is that it is not confined to a hose, so it does not <em>have to</em> pass through the nozzle. And in fact, because the wind turbine at the end of the “nozzle” is trying to slow the wind (remember, that is its job), the wind simply sees the augmented turbine as an obstruction and passes around it rather than going through it—taking the path of least resistance. Although a duct does serve to increase the capture area of a wind turbine, a more effective way to accomplish that goal is with longer blades.</p>
<p>One of the benefits touted for vertical-axis turbine designs, such as the Savonius and Darrieus rotors, is that they can take wind from any direction. While it is true that these designs can be simpler because they do not need moving parts to respond to changes in wind direction, the benefit stops there. Wind only blows in one direction at any given moment. Although wind does indeed change direction, sometimes abruptly, the flow through a vertical-axis turbine takes time to react to this changing flow—even if it is not as evident as with a propeller rotor reorienting itself. The claim that vertical-axis designs are better suited for the directional turbulence experienced on short towers is strictly a myth. With any wind turbine design, efficiency increases with steadier wind flow, which is another reason to place any wind turbine, regardless of type, on a tall tower. </p>
<h2>Physics Rule</h2>
<p>Keep in mind the fundamentals of wind energy physics next time you are evaluating wind turbine claims. There are two points most worth remembering:</p>
<p>All wind turbines extract kinetic energy from the moving air by slowing it down, and those that find the best balance between slowing the wind and not disrupting its flow are the most efficient—up to the Betz limit of 59.3%.</p>
<p>The governing equations are dependent on two factors we have some control over—wind turbine swept area (bigger captures more energy) and wind speed (which is a huge factor because it is cubed in determining power). Use these factors to your advantage when choosing and siting a wind energy system.</p>
<h2>Access</h2>
<p><strong>David Laino</strong> puts his aeronautical and mechanical engineering knowledge to use at work designing wind turbines for Endurance Wind Power, and for fun, sailing on the Chesapeake Bay in Maryland.</p>
</div></div></div>Sat, 26 Apr 2014 23:45:58 +0000Doug Puffer12412 at http://www.homepower.comhttp://www.homepower.com/articles/wind-power/design-installation/wind-energy-physics#comments